This invention relates to gradient index (GRIN) lenses, devices including GRIN lenses and methods for making GRIN lenses.
GRIN lenses are widely used in optical devices and telecommunications applications such as switches, circulators, isolators, and wavelength division multiplexers. GRIN lenses typically utilize a glass rod having a sufficient diameter such that the index of refraction of the GRIN lens can be gradually varied from the axis of the lens towards the outer periphery of the rod.
The trend toward miniaturization of photonic devices has led several manufacturers to offer gradient index (GRIN) lenses with diameters smaller than the industry standard 1.8 mm. Many manufacturers make these smaller diameter lenses by simply starting with smaller-diameter cane, and using an ion exchange process similar to that used for the larger diameter products. This results in the core refractive index, no, and the surface refractive, nr, index being essentially the same as in the larger diameter lenses, so that the gradient parameter,       A    ,      xe2x80x83    ⁢      (          =                        1          r                ·                              2            ·                          (                              1                -                                                      n                    r                                                        n                    o                                                              )                                            )  
is larger for the smaller diameter lenses, where r is the radius of the lens. Since the lens properties (focal length, beam diameter, etc.) depend strongly on the value of {square root over (A)}, the performance of the smaller lenses differs markedly from that of the standard 1.8 mm lenses. In particular, the beam diameter (also known as spot size) is smaller for lenses having larger values of {square root over (A)}. This is undesirable for many applications since the smaller beam diverges more rapidly, has different misalignment sensitivities, and will adversely affect the performance of narrow-band Wavelength Division Multiplexing (WDM) filters.
One way to avoid this problem is to make a standard 1.8 mm lens, then perform a grinding and polishing operation to reduce the outside diameter of the lens. This produces a smaller diameter lens having the same {square root over (A)} value as the standard lens. However, it is undesirable on two counts: it adds a costly grinding step to the manufacturing operation; and it wastes glass by producing fewer lenses for each pound of glass melted and formed into cane. Another potential means to produce small diameter lenses producing beam diameters comparable with the standard diameter lenses would be to alter the glass composition, so that changes in nr and no compensate for the change in r to produce the same value of {square root over (A)}. However, this is undesirable from a manufacturing standpoint, in that it requires different glasses to be produced for lenses of different properties, which adds complexity to the manufacturing process. It is preferable that a single glass could be used for a multitude of different lens products. While it would be possible to utilize a single glass by prolonging the ion exchange step or heat treatment step in lens manufacture to flatten the gradient, thereby reducing {square root over (A)} for the smaller lenses, this is also undesirable from a manufacturing standpoint, since it lengthens the manufacturing time, thereby reducing throughput.
Although there is a particular interest in small diameter lenses having a diameter less than about 1.8 mm, many photonic devices, such as MEMs switches, require longer optical path distances than are provided with small diameter lenses. It would be desirable to provide a lens having a larger beam, hence a longer optical path distance, with a lens having the standard 1.8 mm diameter, since this would allow use of standard packaging and fixtures.
Certain embodiments of the invention relate to graded index lenses having a pitch less than 0.23. Other embodiments relate to devices including such GRIN lenses, such as collimators. Still other embodiments relate to methods of making GRIN lenses having a pitch less than 0.23. Other embodiments relate to lens having pitch of about 0.23-0.25. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.